Wide band feed through stub



Dec. 18, 1951 R; w. MASTERS 2,579,152

WIDE BAND FEED THROUGH swus Filed June 25, 1946 2 SHEETS-SHEET 1 I17. Z 18 2; 20 la E l E i L, Q I a 1 k n J i f," i f, 1', 2'6,

10o x k E& INVENTOR. E i /Pahvf W/Myfem' Q n B I a, '6 w CQM FREQUENCY AY'ZWR/VEI Patented Dec. 18, 1951 "YUNITED STATES PATENT OFFICE WIDE BAND FEED THROUGH STUB Robert W. Masters, Erlton,N. J., assignor .to Radio Corporation of America, a corporation of Delaware Application June 25, 1946, Serial No. 679,075

This invention relates to radio frequency transmission lines and particularly to arrangements for transmission of radio frequency energy between different sources thereof and relatively rotatable equipment respectively associated therewith.

In accordance with the invention as applied, for example, to a system in which two antennas on a common rotatable mount are connected by coaxial type" transmission lines to their respective receivers or transmitters, the two lines are effectively brought into coaxial alignment in the axis of rotation of a double rotary joint by feedthrough stubsand transformer sections which provide for merger of the conductors of the incoming, or outgoing, lines into a three-conductor dual-line with complete avoidance of electrical coupling between the two lines at all wavelengths for which the walls of the conductors act as percfect shields. I

The invention further resides in features of construction and arrangement hereinafter described and claimed.

V For further understanding of the invention and for illustration of constructions utilized, reference is made to the accompanying drawings, in

which: 1 1 a Figure 1 is a cross-sectional view showing in simplified form the construction of a dual rotary joint; and associated networks necessary to bring the lines into concentricity.

Figures l-A and 1-3 are explanatory figures; Figure 2, in cross-section, shows a simplified form of another type of rotary joint; and associated networks necessary to bring the lines into concentricity.

Figure 3, partly in cross-sectiomshows a rotary joint construction for connecting'two antennasrespectively to two relatively stationary radio equipments; and p I Figure 4 is a cross-sectional view showing in detail the construction of one unit of a double coaxial rotary joint.

Referring to Figure 1, it is assumed, for purposes of explanation, that the two concentric lines I and 2 in the lower portion of the figure extend respectively from radio equipments used, for example, for communication, object location or identification purposes, and that their continuations in the upper portion of the figure separately extend to two suitable loads, for example antennas. The radio frequency energy transmitted by line comprising the concentric.

conductors I and His to be kept isolated from the radio frequency energy transmitted by line 5 Claims. (01. 178 -44) 2 2 comprising the concentric conductors i2 and I3. In the lower portion of the figure, the two lines I and 2 merge into a three-conductor line H, I2 and l3-which in the upper portion of the-figure is branched out again to form two concentric lines I and 2. At theaforesaid physical merger of the lines, the outer conductor Ill of line I joins the inner conductor [3 of line 2 and serves as a continuation of it until the branch point is reached. r I

In the vicinity of the merger of the lines, the line 2 is provided with a transformer section T2 whose characteristic impedance is different from that of the adjoining sections of that line;-pr'eferably, within thelimits of the transformer sec-' tion T2, the diameter of the inner conductor 13- of line 2 is suitably enlarged to attain that ratio of conductor diameters which affords the different desired impedance. The length of the transformersection T2 is an even number of quarter wavelengths at a preselected frequency, for example the mean frequency of the band or range of frequencies to be transmitted by line 2,'so that the line sees the same impedance looking into section T from either end thereof. Also in the vicinity 'of the merger of the lines I and 2, there is provided an extension 16 of the outer conductor [2 of line 2 which surrounds the terminalsection I? of the incoming line I; the outer conductor III of line I and the section IB of the outer-conductor of line 2 form a stub S2 of line 2 through which line i extends. Thestub S2 is preferably closed-ended mechanically to support the line I and, in such case, extends an odd number of quarter wavelengths from the merger of the lines. This feed-through stub and transformer arrangement, as well as thoselater. described, provides for physical merger of the two transmission lines yet avoids electrical coupling between them and further adapts line 2 for transmission of radio frequency energy throughout one or more wide bands of frequency with low reflection, that is, with low standing wave ratio. Referring to Figure 1A, for a purely resistive load of fixed magnitude, the broad band stub support arrangement is subtsantially reflectionless at three frequencies, one (in) at the. center of theband and another two on opposite sidesof the center frequency and mathematically related thereto in dependence upon design of the is) of frequencies within which energy is transmitted with low reflection. Curve E is the corresponding characteristic of a similar arrangement aifording a wider band (f1' to In) of frequencies within which energy is transmitted with low reflection but at some frequencies with somewhat higher reflection than for curve C. There is an infinity of such curves, each with di-iferent end points F1 and F20 In general, the wider the separation of the zero reflection frequencies, the greater the reflection at some frequencies within the band. The maxi-- mum band width to be used in a particular case depends upon the reflection which can be tolerated at the worst point in the band: for example.

some transmitting .systems exhibit instability when the reflections in the transmission line ex:- ceed certain fixed values for the systems. 'The stub and transformer combinations would then be designed so that their combined reflection wo ls be es than th permiss ble v ue- Ih up e imit of the baud... Fi u e -V i h frequency n at whi h h stub eeihes a halfwave on a d t e we imi i Zeie u the practica l m s. cons r n e ere e .refl e s eni ab 0. 1% to ab u 1-3 10:

the simplest iorm of feed-through, broadbehdiu a ran ment th tu she l as S2 of Fi ure l. s onea t wa e th lens t m band frequency and the transformer section, such as T of F ure l. is Que-hal W e e t l s at th t frequen y and s s mm ca i h s e to the stub- .h Q19 e61l 6 stub m y be. a y odd nu f qua er wav l n hs ns an e transformer section s pr e ably t i e the t b le t and centrally located. In any event, for the longer transformer sections the median line of the stub hould he a d numb of ua er a el ng rom eaeh e o he ransf rmer etien- S rran ements imi the-perm s le band width but. on he other a d. as ndi at d by eurve D of Figure 1-13, provide for, two or more band pass an es in harmonic elation h p- A ematica am; is, o he netwerls iselesesth t. ii he i 11 in sym m is d s hat d n an? ed t Fi ure l Zo -characteristic impedance of line 2 as well as the stub section comprised of conductors I6, and I1,

Z1=characteristic impedance of'the transformers "T1 and T2,

we find the relationship,

Zr n

where r h. r i 2 ,2fo

and

)i wavelength at center band (jo) and wavelength (variable) corresponding to frequency (,f).

This equation (1) governs. the behavior of the stub and transformer combination. To design one, f1 or f2 and in are selected, and p is determined. Then insert the value of p into (1) and solve the resulting cubic equation in Q2 Zn.

for

Knowing in advance, the value of Zn, there is obtained the necessary value for Z1. It will be found that ii and f2 will be removed from )o by equal percentages of I The band is then investigated for reflection by other means either mathematical or experimental.

-Reverting to specific discussion of Figure 1, from the stub S2 to the upper branch point, the two lines I and 2 are formed by the three conductors II, I2 and I3, the last-named serving both as the outer conductor of line I and the inner conductor of line 2. The elements thus far described below the line A-A are comprised in one member or unit I5 of a rotary joint construction.

The other, relatively rotatable unit I4 of the joint may be of identical construction and need not be described in detail. In brief, above the dot-dash line A.A, which may be considered a dividing plane between the two relatively rotate able parts of the joint, there. are also three concentric conductors .II, 12 and I3 which are 00-. axially aligned with respect to the axis of rota,- tion B--B of the joint. These three conductors, electrical continuations of the same conductors of the lower joint member I5, extend to the point at which the two lines as formed by three .0011? ductors branch out into the two separate. outgoing lines having a total of four conductors.

In the vicinity of the branch point, the line 2, is provided with a transformer section T1 similar to transformer section T2. It may be formed. for example, by enlargement of the diameter of conductor I3, and in cooperation with a stub section SI provides for 10w reflection of energy throughout a broad band of frequencies. The stub SI, similar to stub S2, is formed by the upper extension I6 of the outer conductor I2 of line 2 which surrounds the line I for a distance from the branch point which is an odd number OI quarer wavelen ths. In the preferred construc i thestub SI is a closed=ended stub hav an elecri a e h oi one-Quarter,w velen h. This. provides the simplest construction, affords me: hanieal support fthe section Il of the n r lin n l o p rmits the space b tween the eene ues o l ne to. be isola ed from. t e ambientat- I I, I2 and I3 shall be in contact to maintain their electrical continuity during relative rotation of the two units of the joint. The details of a construction providing such continuity will be dis.- cussed in description of- Figure 4. A

In the modification shown in Figure l, the. line 2 in eifect continues straight through they rotary joint without any bends or change in direction. In some installations, it may be necessary or desirable to have bends in both lines adjacent their physical joinder or separation at the rotary joint. A construction meeting this requirement for both joint units is shown in simplified form in Figure 2. Except in particularsspeciflcally discussed, the physical and electrical relations of the conductors of the lines to and from and within the. joint are. the same as above specifically discussed in connection with Figure 1.

Referring to the lower unit 23 of the joint, Figure 2, both lines I and 2 extend horizontally toward their merger point I where they turn upwardly through a right angle and merge into a three-conductor line concentric with the axis of rotation B of the joint. Above the dividing line A-A between the tworelatively rotatable units 22, 23 of the joint, the three-conductor line continues to the branch point at which it divides into the outgoing horizontal sections of lines I and 2.

More specifically, at the bends where line 2 enters and leaves the rotary joint, it is provided with transformer sections T3 and T4, each comprised in part in the horizontal section ofthe line and in part in the vertical section. Preferably, each bend is midway of, the associated transformer, section, or if the section is more than a half wavelength, at an odd number of quarter wavelengths from the ends. of the section.

The joint units ,arealso provided ,with feedthrough stubs s3 and $4, each formed by a continuation, beyond the bend, of the outer conductor 26 of the horizontal portion of the larger line 2 and the outer conductor I8 of thatportion of the smaller line I surrounded thereby. More specifically, the lower stub S3 is formed by the lower extension 24 of outer conductor of incoming line 2 and the terminal portion of the incoming line I; the upper stubS l is formed by the upper extension 24 of the outer conductor 20 of outgoing line 2 and the initial portion 25 of the outgoing line I. Preferably the stubsare short-circuited or closed-ended mechanically to support the inner line and to provide a gas-tight seal permitting theline 2 to be filled with a dry inert gas at super-atmospheric pressure.

With short-circuited v stubs, their electrical length should be an odd number of quarter wavelengths. Preferably, the stubs S3, S4 are each one-quarter wavelength long and the transformer sections Ts'and T4 with which they co-act are a half wavelength long. As in the type joint shown in Figure 1, the feed-through stubs and the associated transformer sectionsprovide for physical merger of the lines without introduction of electrical coupling between them and co-act, so far as line 2 is concerned, over a wide band of frequencies so suiting the arrangement for operation with radio equipment 'whose signal energy, as in object-location .or television, requires a wide band of frequencies, or which at different times is tuned to different frequencies of a band. 7

' It is feasible to use a double coaxial rotary joint construction whose upper (or lower) element is of the straight-throughtype shown in Figure 1 and whose lower (or upper) elementis of the bend type shown in Figure 2. Referring I, to Figure 3, for example, the lower rotary'joint member 23, of type shown in Figure 2, is fastened as by mounting plate 33 interiorly of the cylindrical base member 3I of a pedestal mount upon whose rotatable cap member 32 are secured two directional antenna systems, generically exemplified by 28 and 29, and respectively 'connected by the concentric transmission lines I and 2 to the radio equipments 26 and 21-.

The upper member or unit I4 of the rotary joint, of type shown in Figure 1, is fastened as by a mounting plate 34 to the rotatable member "Referring to Figure 4 which shows in detail" the construction of a rotary joint of the type shown in Figure 2, the body member 35, of which the mounting late 33 may be an integral, part, is in the form of a hollow T joint whose open horizontal ends are recessed to receive the tubular sections 36 and 49, the former serving as the outer conductor of a feed-through stub and the latter as the outer conductor of transmission line 2.

The closed or short-circuited end of the stub is formed by cap member 31 suitably recessed to receive the tube 36. The closure member 31 is provided with a central aperture or bore which at one end receives the outer conductor .38 of transmission line I and which at the other end receives and supports the tube 39 which serves as a continuation of the outer conductor of line I and as the inner conductor of the stub.

The other end of tube 39 is received by an open end of the inner T member 40 which, as hereinafter appears, forms the central part of a trans former section of line 2 and as a continuation of the outer conductor of line I.

The inner conductor II of line i, supported centrally of the outer conductor 38 by a series of insulating discs or washers 42, is provided with a terminal plug 43 received by the socket end of conductor 44, which serves as a continuationof the inner conductor of line I. The ratio of the outer diameter of conductor 44 to the inner diameter of tube 39 which surrounds it is the same as the ratio of the corresponding diameters of conductor M and tube 38 so that the character- .istic impedance of line I remains unchanged: for

like reason, the opening in plug 31 and the end of conductor 4| are smoothly and similarly tapered.

The left end of conductor 44 is provided with a socket to receive the plug 46 extending'from member 41a which serves as a further continuation of the inner conductor of line I. The left end of member 47a threadably receives the lower end of the vertically extending member 471) which is in alignment with the axis of rotation B-B of the joint and serves as a further continuation of. the inner conductor of line I. The sleeve member 48 which fits over the tubular vertical extension of member 40 serves both as a continuation of the outer conductor of line I and, as hereinafter appears, also of the inner conductor of the larger line 2.

The inner conductor 50 of line 2 is supported centrally of the outer conductor 49 by the spacer discs 5| of suitable insulating material and at its termination is slidably received by the socket end of member 52 which serves as a continuation thereof. A bolt, as shown, may be provided to secure these two parts to each other. The other end of member 52 receives the left end of the inner T joint member 40 and with it forms part of a transformer section, as will be understoodfrom previous discussion of Figure 2.

As thus far described, the construction pro-'- vides for merger of the four conductors of the horizontal extensions of lines I and 2 into threevertically extending concentric conductors in coaxial alignment with the axis of rotation of the joint; at this part of the joint, the three concentric conductors are members 471), 48 and 35 which, as now described, are in alignment with and respectively engage three similarlyeoncentric conductors which rotate with respect thereto stub and transformer construction-there downwardly extend the a. three concentric coriduclms 61x63 and 55, the first-named serving'as the inner conductor of line 4, the second-named serving both as theouter conductor of line I and as the inner conductor of line 2, and the last-named serving as the outer conductor of line 2.

The stationary member .35 of the lower joint unit engages the lower face of the relatively rotatable member 53 "to the upper end of which-is secured the tubular concentric member 54 which receives the lower end of the outer vertical eoncentr-ie conductor =55. Consequently members 53, *54 and 55 serveas the outer conductor of that part of line 2 which is rotatable about axis BB of the joint. r

Reverting to the physically common conductor of the two lines I and "-2, the sleeve 56 which receives the upper end of above-mentioned sleeve 48 and the lower end of sleeve 57 serves electri ea'lly to connect 'them during their relative -rotation': preferably sleeve 56 is free to rotate about both of the other members, the areas of overlap and closeness of fit insuring 'low loss of radio frequency energy transferred despite use of a light lubricant on the bearing surfaces oithe sleeves.

'The upper end of sleeve 51 receive and is secured to the lower end of tubular 'mem'berfi'ii which forms the last part of the transformer section beginning with member 52. The upper end of member 58 received the "lower end of condoctor 63 which forms the further continuation of the outer conductor of line i and of the inner conductor ofline'Z.

Beginning with member llb, the vertical continuation of the inner conductor of line i includes the member 159 whose lower plug end is rece'ivedby the upper socket end of member 411), the sleeve .60 which receives the upper plug end of member 59 and thelower plug end ,of member 61, the member 6| whose upper plug end is received by the lower socket end of the central condoctor '62. The .upper plug end of member BI is preferably rigidl connected to the lower socket end of member .62 as by silver solder and the upper socket endof member 5B is preferably rigidly connected to the lower plug .end of member 6'! as by a tight press fit. There is thus circumvented possibility of annealing of the spring fingers of the lower socket end of member Bil which engage and rotate with respect .to the upper plug end of member 59.

.As shown, the proper spacing and alignment of conductors 55, .52 and 53 are preserved by the insulating spacers 45 and 5!. The relatively movable parts of .each conductor .of the threeconductor line overlap :suiiiciently to insure .continuance of alignment during rotation.

The pipe connectionM in :the stationary mem her 35 is for introduction of inert gas into the space between the inner and outer conductors ifif line 2. The construction of the relatively rotatable parts 35 and -5-3 is such :that escape of :g-as is minimized; however, the sealing features at that point are not specifically part 201" :thepresent invention and are-not disclosed.

It shall be. understood the invention is not limited to the particular arrangements rdisclosed' but includes changes and modifications within the scope of the appended claims.

I claim as my invention:

.1. A dual rotary joint construction providing for transmission of radio frequency energy over a band of wavelengths between relatively rotatable portions of each of two concentric lines .comprising two :relativelyrotatable units each including three concentric conductors coaxially aligned with respect to the axis of'rotation of the joint and connected respectively to the corresponding conductors of the other unit, each of said units comprising an arrangement for merging relative ly fixed portions of the two concentric transmisr sion lines with said three conductors whose inter mediate conductor serves both as the inner conductor of the first of said lines and as the outer conductor of the second of said lines, the -relatively rotatable portions of the first of said lines each having a transformer section whose electrical length is an even number of quarter wavelengths substantially midway of said band, and each -of said units including a feed-through stub for the second of said lines, the inner conductor of the :stub being the outer conductor of the second line and iio'ining the inner conductor of the transformer section of the first line at an odd number of quarter wavelengths from each end of that section, and the outer conductor of the stub being an extension of the outer conductor of the first line and whose electrical length is an integral number of quarter wavelengths, said stubs and transformers *therebyeoacting to mime miz-e electrical coupling between said lines throughout said band-of wavelengths.

2. A dual rotary joint construction providing for transmission of radio frequency energy over a band of wavelengths between relatively rotatable portions-of-each-of two concentric lines without introduction of eleetrical'coupling betweenthe lines comp-rising two relatively rotatable units each including three concentric conductors coaxially aligned with respect to the axis of rot-a- 1 tion of the joint and connected respectively to the corresponding conductorsof the other unit, the outermost and innermost conductors of each unit serving respectively as continuations of the outer conductor of the first of said lines and the inner conductor of the second of said lines and the intermediate conductor serving as a continuation of both the inner conductor of the first of said lines and the outer conductor of the second of said lines, said first line adjacent its joinder with each unit having a transformer section whose characteristic impedance is lower than that of adjoining sections and a closed-ended extension of its outer conductor which surrounds e s cond line in thevicinity r its .joinder with the unit and forms therewith a feed-through stu extension of thefirst line, said tr n former sections eachbeine an integral number .of half wavelengths long,.said closed-ended feed-through stubs each being an odd number of quarter wavelengths long .and extending from the .,corresponding transformer section at an odd number of .guarter wavelengths iromeachend thereof ,.so to minimize electrical coupling ,betweensaid lines throughout said band of wavelengths.

3..A dual rotary joint construction providing for transmission lofradio ,irequency energy over a .band f wavelengths between relatively rotat able portions .of each of two concentric lines without introduction of ,electrical coupling between the lines gcomprising .two relatively rota-table units each includi g i hlieelconceritric conductor .coaxially aligned with respect to the axis of rotation of the joint and comectedkrespewtively to the corresponding conductors .of the other unit, the outermost and innermost conductors of each unit serving respectively a continuations of the outertonductor .of the first of said lines and the inner conductor of the second of said lines and the intermediate ;-c0nducto.r seri ing-asacontinuationof both the inner conductor of the first of said lines and the outer conductor of the second of said lines, said first line having adjacent its connection with each unit a transformer section whose characteristic impedance is lower than that of adjoining sections and whose electrical length is an .even number of quarter wavelengths, and also having adjacent its said connection a closed-ended extension of its outer conductor which surrounds the second line for an odd number of quarter wavelengths and forms therewith a feed-through stub extension for said second line electrically coacting with said transformer section of said first line in avoidance of electrical coupling between said lines throughout said band of wavelengths.

4. A dual rotary joint construction providing for transmission of radio frequency energy over a band of wavelengths between relatively rotatable portions of each of two concentric lines without introduction of electrical coupling between the lines comprising two relatively rotatable units each including three concentric conductors coaxially align-ed with respect to the axis of rotation of the joint and connected respectively to the corresponding conductors of the other unit, the outermost and innermost conductors of each unit serving respectively as continuations of the outer conductor of the first of said lines and the inner conductor of the second of said lines and the intermediate conductor serving as a continuation of both the inner conductor of the first of said lines and the outer conductor of the second of said lines, said first line having adjacent its connection with each unit a transformer section whose characteristic impedance is lower than that of adjoining sections and which is a half Wavelength long and also having adjacent its said connection a closed-ended extension of its outer conductor which surrounds the second line for one quarter wavelength and forms therewith a feed-through stub extension for said second line and electrically coacting with said transformer section of said first line in avoidance of electrical coupling between said lines throughout said wide band of wavelengths.

5. A coaxial line arrangement for transmission of radio frequency energy between sources thereof and relatively rotatable equipment respectively connected thereto by coaxial lines each having an inner and an outer conductor comprising two relatively rotatable rotary-joint units each including three concentric conductors at least in part coaxially aligned with the axis of rotation of the joint for connection respectively with the corresponding conductors of the other unit, the intermediate conductor serving both as the inner conductor of the first of said coaxial lines and as the outer conductor of the second of said coaxial lines, said first of said lines including half-wave transformer sections, one for each unit, and closed-ended quarter-wave stubs, one for each unit, each comprising an extension of the outermost of said three concentric conductors and a section of the second of said lines which extends therethrough for connection with the other two or said three concentric conductors. said stubs each mechanically supporting the associated section of the second line and extending from the center of the corresponding transformer section of the first line to attain a low standingwave ratio over a broad band of frequencies and negligible electrical coupling between said lines throughout said band of frequencies.

ROBERT W. MASTERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,223,224 Newhouse Nov. 26, 1940 2,426,633 Mason Sept. 2, 1947 2,446,982 Pound Aug. 10, 1948 OTHER REFERENCES Principles of Radar, M. I. T. Radar School, The Technology Press, Massachusetts Institute of Technology, 1944, pages 8-112 and 8-115. 

